Pluto, Charon, Nix, Hydra, Kerberos, and Styx

compiled by Wm. Robert Johnston
last updated 13 July 2013

Orbital elements and other data (Assumed or derived values in parenthesis, my estimates in italics and parenthesis. Source identifications in brackets, see this link for sources):

orbital data, primary (osculating elements) [JPL]:
semimajor axis a: 39.48168677 AU
orbital period P: (248.08102 y = 90,612.5 d)
eccentricity e: 0.24880766
perihelion distance q: 29.6583407 AU
aphelion distance Q: 49.3050329 AU
inclination i: 17.14175°
argument of perihelion omega: 224.06676°
ascending node OMEGA: 110.30347°
mean anomaly M: 238.92881°
perihelion passage TP: 1989-09-05
Epoch: 01 JAN 2000
data arc: 1930-?

orbital data, Charon:
semimajor axis a: 19,571.4 ± 4.0 km [B06d]
semimajor axis/primary radius a/Rp: (8.36)
orbital period P: 6.3872304 ± 0.0000011 d [B06d]
eccentricity e: 0.000000 ± 0.000070 [B06d]
inclination i: (0.00°)

orbital data, Nix:
semimajor axis a: 48,675 ± 21 km [B06d]
semimajor axis/primary radius a/Rp: (20.80)
orbital period P: 24.8562 ± 0.0013 d [B06d]
eccentricity e: 0.0023 ± 0.0021 [B06d]
inclination i: 0.10° ± 0.33° [B06d]

orbital data, Hydra:
semimajor axis a: 64,780 ± 88 km [B06d]
semimajor axis/primary radius a/Rp: (27.68)
orbital period P: 38.2065 ± 0.0014 d [B06d]
eccentricity e: 0.0052 ± 0.0011 [B06d]
inclination i: 0.25° ± 0.11° [B06d]

orbital data, Kerberos:
semimajor axis a: 59,000 ± 2,000 km [C2769]
semimajor axis/primary radius a/Rp: (50)
orbital period P: 32.1 ± 0.3 d [C2769]
eccentricity e: ~0? [C2769]
inclination i: ~0? [C2769]

orbital data, Styx:
semimajor axis a: 42,000 ± 2,000 km [I9253]
semimajor axis/primary radius a/Rp:
orbital period P: 20.2 ± 0.1 d [I9253]
eccentricity e: ~0? [I9253]
inclination i: ~0? [C2769]

other data, primary:
diameter: 2304 ± 13 km
2412 ± 22 km
absolute magnitude H: (-1.0)
rotation period: (6.3872304 d)
amplitude delta M: -
color index U-B: (U-B variable with rotational phase and year-to-year:)
0.20±0.01 to 0.35 ±0.05 (in 1980-83) [L84a]
0.20-0.23 (in 1978-82) [A92a]
0.34 (in 1988) [A92a]
color index B-V: (B-V variable with rotational phase and year-to-year:)
0.82±0.01 to 0.87±0.02 (in 1980-83) [L84a]
0.866±0.007 (in 1987) [M92a]
0.82 (in 1988) [A92a]
0.868 (in 2006) [B06d]
color index V-R: -
color index V-I: 0.93
slope parameter G: -
geometric albedo: 0.6 (regional, 0.3-0.8)
mass: (1.305±0.007)x1022 kg [B06d]
density: 1.92 ± 0.12 g/cm3 [P06a]
gravitational acceleration:
(surface):
(0.62 ± 0.04 m/s2)

other data, Charon:
diameter: 1,212.0 ± 3.0 km [P06a]
oblateness : 0.006 ± 0.003 [P06a]
diameter ratio Ds/Dp: (0.518)
absolute magnitude H: (0.9)
component magnitude difference: (1.9)
rotation period: (6.3972304 d)
amplitude delta M: -
color index B-V: 0.702±0.010 [M92a] 0.710 [B06d]
color index V-R: -
color index V-I: 0.83
slope parameter G: -
geometric albedo: 0.37± 0.02
mass: (0.152±0.006)x1022 kg [B06d]
mass ratio Ms/Mp: 0.1165 ± 0.0055 [B06d]
density: 1.63 ± 0.07 g/cm3 [P06a]
gravitational acceleration:
(surface):
(0.28 ± 0.01 m/s2)

other data, Nix:
diameter: (40-90 km?)
diameter ratio Ds/Dp: (0.02-0.05?)
absolute magnitude H: (8.64 ± 0.09) [B06d]
component magnitude difference: -
rotation period: ?
color index B-V: 0.91 ± 0.15 [B06d]

other data, Hydra:
diameter: (45-110 km?)
diameter ratio Ds/Dp: (0.02-0.05?)
absolute magnitude H: (8.48 ± 0.10) [B06d]
component magnitude difference: -
rotation period: ?
color index B-V: 0.64 ± 0.12 [B06d]

other data, Kerberos:
diameter: 14-40 km [C2769]
diameter ratio Ds/Dp: ?
absolute magnitude H: (11) [C2769]
component magnitude difference: ?
rotation period: ?
color index B-V: ?

other data, Styx:
diameter: 10-25 km [I9253]
diameter ratio Ds/Dp: ?
absolute magnitude H: (11) [I9253]
component magnitude difference: ?
rotation period: ?
color index B-V: ?

--Pluto--discovery and notes:

Pluto was discovered 2 March 1930 by Clyde Tombaugh from the Lowell Observatory, Arizona, USA. The search resulting in its discovery was motivated by Percival Lowell, seeking to identify a hypothetical "Planet X" producing gravitational perturbations on Neptune's motion. Early estimates of Pluto's diameter were in the range 4,000 km to 12,000 km; only with the discovery of Charon was Pluto's size recognized as about 2,400 km. With the discovery of trans-Neptunian objects beginning in 1992, Pluto was then recognized as one of the larger members of the class of TNOs. It was classified as a planet from its discovery until 24 August 2006, when the IAU created a category "dwarf planet" to include Pluto; on 7 September 2006 the MPC assigned minor planet number 134340 to Pluto.

Charon was discovered by James W. Christy on 22 June 1978 from observations from the U.S. Naval Observatory over the period 29 April 1965 to 12 May 1978. Provisional designation S/1978 P1. Among satellites of planets or dwarf planets, Charon is larger with respect to its primary than any other.

Nix was discovered by H. A. Weaver, S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young, and L. A. Young, using Hubble Space Telescope observations of 15 May 2005. Provisional designation S/2005 P2; permanent name assigned in 2006.

Hydra was discovered 15 May 2005 by H. A. Weaver, S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young, and L. A. Young, using Hubble Space Telescope observations of 15 May 2005. Provisional designation S/2005 P1; permanent name assigned in 2006.

Kerberos was discovered in images obtained 28 June, 3 July, and 18 July 2011 by the Hubble Space Telescope by M. Showalter, A. Stern, H. Weaver, A. Steffl, and L. Young. Subsequently identified in prediscovery observations from 15 February 2006 and 25 June 2010. Discovery announced 20 July 2011. Provisional designation S/2011 (134340) 1; permanent name assigned 2 July 2013.

Styx was discovered in images obtained 26 June, 27 June, 29 June, 7 July, and 9 July by the Hubble Space Telescope by M. R. Showalter, H. A. Weaver, S. A. Stern, A. Steffl, M. W. Buie, W. J. Merline, M. J. Mutchler, R. Soummer, and H. B. Throop. Provisional designation S/2012 (134340) 1; permanent name assigned 2 July 2013.

Mutual eclipses and occultations by Pluto and Charon occurred from 1985 to 1990, permitting determination of various physical data. Pluto was confirmed to have a thin atmosphere in 1988, including nitrogen, methane, and carbon monoxide.

--Links, on this site:

--Links, more technical:

--Links, less technical:

--Links to ADS abstracts:


© 2006-2011, 2012 by Wm. Robert Johnston.
Last modified 13 July 2013.
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